Think you know how many calories you get when you order a “low-fat” sub? Or how many ounces are in the soda you guzzle down at the movies?

Whatever you think you know, you’re likely wrong, according to Pierre Chandon, professor of marketing at the graduate business school INSEAD in France, and a visiting scholar at Harvard Business School. Chandon, who conducts research on food marketing and people’s food perceptions and preferences, spoke at Harvard School of Public Health on Monday, March 19, 2012.

“People often underestimate portion sizes,” Chandon said. If a cup of soda is labeled “extra small,” people think it’s not very much—even if it really isn’t that small, he said. If food is labeled “low-fat,” people tend to eat more of it, thinking it will be less caloric—often failing to consider that low-fat doesn’t necessarily mean low-calorie.

Chandon said people tend to assess how much food they’re eating or how “caloric” it is by the way it looks, often regardless of what the packaging might say. But the way food or drinks look can be misleading.

Chandon gave some compelling examples of this “portion distortion,” based on recent research.

Healthy is not necessarily “low-cal”

In a 2005 study led by University of Scranton psychologist Michael E. Oakes, people were asked which is better to snack on to keep from gaining weight—a “mini” Snickers bar or a cup of 1%-fat cottage cheese with three carrots and three pears. Most people chose the latter. The actual calorie count? Mini-Snickers bar, 47; cottage cheese, carrots, and pears, 569.

Volume is difficult to estimate

Chandon and colleague Nailya Ordabayeva, in a 2009 study, found that people tend to underestimate food or drink amounts when the containers or cups they’re in expand in three dimensions (height, width, and length). For instance, shown two mugs—one that widens substantially near the top, and another that’s taller and thinner—study participants had trouble discerning that the shorter, wider mug actually held more liquid than the taller mug. They were better able to accurately estimate differences when the mugs were the same width, with one taller than the other. This experiment “really shows how hard it is to estimate volume,” Chandon said.

The magically shrinking meal

In a 2010 study by Chandon and colleague Alexander Chernev, participants were asked to estimate calories for one unhealthy item (a cheeseburger) and one healthy item (a salad)—first separately, then together. When they considered the items separately, they estimated the cheeseburger at 597 calories and the salad at 89—a total of 686 calories. But when they considered the items together, they estimated a total of just 511 calories—as if, Chandon said, adding a healthy item to a meal somehow “magically” reduces its overall calories.

Given how hard it can be to accurately assess food amounts and calorie counts—and how this can contribute to the obesity epidemic—Chandon suggested that researchers and policy makers consider not only people’s self-reported beliefs and preferences regarding food, but also their actual preferences and behaviors, as well as new ways to accurately package and label food and drinks.